1. Field of the Invention
The present invention concerns a method and a device for coding the energy of the vocal signal in vocoders with very low throughputs.
It applies notably to the making of linear prediction vocoder devices of the type described in the "Revue Techniques Thomson-CSF" Vol. 14, No. 3, Sept. 1982, pages 715 to 731 and vol. 15, No. 2, pp. 495-516, published by MASSON, 120 Boulevard St. Germain, 75280 Paris.
2. Description of the Prior Art
In these devices, the vocal signal is cut up into time sections or windows with fixed lengths of about 20 milliseconds in the emission vocoders, and each signal window is analyzed to extract the parameters needed for the control of the digital filters of the reception vocoders. These parameters consist of the control coefficients of the reception filters, the r-m-s value of the vocal signal and an indication on whether on the nature of the vocal signal, whether it is voiced or not.
For the digital coding of the speech signal so as to ensure very low throughput links, typically of less than 1000 bits/s, the method for coding the r-m-s value parameter consists in quantifying this r-m-s value parameter on 32 values (0 to 31) according to a logarithmic scale standardized by the NATO standard "STANAG 4198", pertaining to order 10 linear predictive coding, a description of which is given in the article by M. TREMAIN, entitled "The Government Standard Linear Predictive Coding Algorithim-LPC10" published in the journal Speech Technology, April 1982, pages 40-49.
The r-m-s value signal quantified is then coded on 11 bits during three consecutive windows. The r-m-s value of the middle window is coded on five bits and that of each of the farthest windows is coded by a differential coding method on three bits with respect to the r-m-s value of the median window. A description of this coding method can be found in an article published by Wong D., Juang, B. H., Gray A. H., "An 800-bits/s Vector Quantization LPC Vocoder" in IEEE Transactions on ASSP Vol. 30, 1982, pp. 770 to 780.
However, the encoding on 11 bits of the r-m-s value parameter limits the possibilities of reducing the throughput rate of vocoders, notably to low throughput rates of less than 800 bits/second.